The health and safety of workers using respirators is critically dependent on the performance of these devices. Accordingly, an assessment of the contaminant adsorption properties and associated service life of respiratory cartridges is an important factor in the effective, safe use of this protective equipment. The efficacy of respirator cartridges depends on the properties of the solid sorbent, the characteristics of the contaminant, and the relative humidity of the environment. The proposed research involves the application of a practical theoretical model to the investigation of the contaminant breakthrough characteristics of respirator cartridges which employ solid sorbents. Previously, we developed and applied a theoretical model addressing respiratory cartridge contaminant breakthrough. The model was applied to a limited number of selected sets of experimental data, each set involving a single contaminant. In the proposed project, the theoretical model will be extended and applied to other experimental data sets in an effort to derive the important theoretical parameters which characterize the respirator cartridge breakthrough properties of approximately 100 additional important environmental contaminants. Currently, these theoretical parameters are not known, and an original research effort is required to derive them. Extensive experimental data relative to the 100 contaminants of interest have been generated, but this data set has not been systematically characterized with a unifying theoretical model. The presentation and validation of such a model and the derivation of pertinent theoretical parameters would provide a tool which could be used by the practicing industrial hygienist to address a very broad spectrum of respirator cartridge breakthrough problems. The theoretical model will be extended and modified in the proposed research study to predict the effect of water vapor (relative humidity) on contaminant breakthrough. The modified theory will be evaluated with respect to its ability to assess the effects of humidity on respirator cartridge service life using approximately 200 different experimental data sets. The data sets are available in our laboratory; they address seven different contaminants and a wide range of test and pre-test conditioning values of relative humidity. The significance and and practical importance of the proposed research is found in the potential to predict with accuracy the performance and service life of respirators used under a wide variety of conditions.